Geochemistry and geochronology of granitoids in the Kibi-Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana.

Abstract

In Ghana the West African Craton is represented by Birimian and Tarkwaian rocks with extensive gran itoid bodies. Granitoids from Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana were analysed for major and trace element contents and found to be characterised by highly-fractionated REE, enrichments, in LILE, and depletion in Nb, Ta and Sr. The LILE enrichment relative to strong Nb–Ta depres sion, indicates that these granitoids were emplaced in an active margin. Based on field relations, geo chemical composition and geochronological data, the granitoids from the Kibi-Asamankese area can be divided into three types, namely; the Eburnean biotite granodiorite (2133–2127 Ma) and hornblende granodiorite (2147 Ma), and the Pre-Eburnean gneissic biotite granite (2193 Ma). The geochemical data of the studied rocks plot in the tholeiitic field, whereas on the A/CNK–A/NK diagram, they generally fall within the metaluminous field, with A/CNK values between 0.69 and 0.88. U–Pb dating of zircons in the granitoids yielded ages ranging from 2193 to 2127 Ma, which are among the oldest ages obtained from the granitoid plutons in Ghana. Such high-precision geochronological data indicate that magmatism occurred over a time-span of about 70 Ma. This provides further evidence that the period 2.1–2.2 Ga was one of the important stages of Birimian magmatism that led to the generation of the granitoids. From the above-mentioned ages, it is possible to link the geological activities to crustal processes and establish the cyclic geotectonic evolution in the West African Craton over time as part of an arc-back-arc basin system.